Health isn't just predicted by how many resources people have, but by how they relate to other people. Feeling isolated make you sick. The social world remodels your body. Your relationships with the people around you profoundly influence whether you perceive the world as a safe, comfortable place, or as one that is threatening and uncertain. As long as your brain thinks that everything is fine, your body is going to be running a program of long-term investment, general maintenance, and rebuilding. If your brain perceives that you are in an uncertain or threatening environment then it's going to activate stress responses in the rest of the body that trigger changes in gene expression. When we feel threatened our fight or flight response will be activated.
A naturally shy, introverted child may hang back on the playground, get picked on for doing so, and become even more likely to withdraw. These variations in individual temperament are relatively small difference to start off with, but they can propagate into big differences over time by shaping the pattern of social choices you make. Consistently choosing one way over the other solidifies your outlook on whether other people are generally good and trustworthy, or unfriendly and threatening. If your worldview leans toward the later, you are more likely to build a more autonomous, independent life for yourself. That in turn propagates this perception that the world is a distant place, that there is no one else you can turn to. That world view seem to be correlated with long term increases in health risk.
The lonely people's immune systems tended to over express an array of inflammation genes, which control immediate tissue repair processes but also drive the wear and tear we know as aging.